Hmm odd... I should try them again...
I specifically asked them what the max input voltage the regulation circuit could handle was, and got no response... they should EASILY be able to go ask an engineer what voltages it can handle.
Well, anyways, I would feel safe running a 17670, which is the cell that fits... it's not going to overdrive the circuit, no way, no how... it's designed to run on 6V after all. It will not likely go into regulation, unless it's a boost as well as a buck circuit, but very likely to direct drive the LED, I saw no noticeable difference in light output with my 17670. I do not have any rechargeable 123's laying around... not sure what the mAh rating of 123primaryies are, but the 17670 is definitely a higher capacity cell then the RCR123's I find on Light hound.
From the tail cap I got around .5A draw on my 17670 lighting up my L2 which is under spec for a Lux3, and get around .9A with the 123's primaries. So it would appear the 17670 is direct driving the LED likely underspec
I can't find a way to get to the LED contacts to test for Voltage and Current at the LED, but I'd feel pretty safe running it off an 17670, since it's not pulling enough current at the battery to over power the LED, It's likely just on the edge off falling out of regulation, just as if the primary 123's were dying, or is going right into direct drive.
Thats too bad though, i was hoping Pentagon would have responded with a Yes or No with a positive answer, sounds like they don't even fully understand what's being asked and that a 17670 will not provide the same voltage as 2 cr123's
Interesting on the upgrade, they do seem to try and make things compatible, since you can buy any of the 6V heads, either Xenon or the multi-Led's such as the UV (might be on my list of heads to add) all can go on the L2 assembly.
I didn't see anything in the online warranty/garantee about the use of rechargables voiding the warranty or anything, in fact there was something about wether rechargables were covered, but I see no rechargable lights in the product line.